Vitamins and the prevention of diseases.

Faith in vitamin E, β carotene, and oestrogen endures despite the evidence

Throughout the 1990s there was a widespread believe that vitamin E could help prevent cardiovascular disease. Large observational studies found clear evidence of benefit. Since then. equally large and more convincing randomised trials have found the opposite.
Belief in the protective properties of vitamin E persists. In scientific articles citing the early observational studies, some half were still arguing in favour of vitamin E as late as 2003 despite there being a landmark trial five years earlier overturning the observational studies findings..
Belief in the anti-cancer potential of β carotene has also persisted. The notion that β carotene prevents cancer was discredited by randomised trials published in 1996. In another citation analysis, two thirds of citations to the original observational work were still favourable 10 years later. Most of the articles simply ignored the later evidence. Results were similar for the more recently discredited idea that oestrogen protects post-menopausal women against dementia.
In the vitamin E analysis, articles in specialist journals were more likely than articles in general journals to defend the protective properties of vitamin E. So were articles that reported their own observational data, those from outside the US. and those that failed to cite the key (negative) randomised trial.
BMJ 2007, Faith in vitamin E, β carotene, and oestrogen endures despite the evidence . vol 335 p 1234
Quoting a review in JAMA 2007;298:2517-26
It is interesting that vitamins may in an epidemiological study be shown to protect against all manner of diseases. But vitamins may be a marker of other nutritional constituents.
One area where there is a better basis for believing that there is protective role for a vitamin is the use of folic acid to reduce the incidence of neural tube disorders. The UK Food Standards Agency has recommended the fortification of flour with folic acid. Here there is a clear evidence of efficacy. Never the less there is continued dissent because of a misinterpretation of trial data. The suggestion is that folic acid fortification increases the incidence of colonic polyps The results do not show this.
The real problem is that vitamins are present in food in minute amounts and act in minute amounts.
What is being asked is that they also have a pharmacological action ie at an increased dosage.
Such claims have to be judged by the same criteria as a pharmacological agent . Backed by Clinical trials. The financial reward to Drug Firms would be so small as to rule out such trials
Clearly this is the province of Research Councils.
Bayston et al 2007, Folic acid fortification and cancer risk The Lancet vol 370, p 2004

Martin Eastwood


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